Rapid development of electronic, communication and computer industries has brought about remarkable development of camcorders, cellular phones, notebook computers and the like. Accordingly, demand for lithium secondary batteries as power sources capable of driving portable telecommunication equipment is gradually increasing. In particular, research and development associated with applications such as electric vehicles, eco-friendly power sources, uninterrupted power supplies, electrically-drive tools and artificial satellites is underway in Korea as well as Japan, Europe, the U.S. and the like.
Cathode active materials for lithium secondary batteries generally use lithium cobalt oxide (LiCoO2), but lithium nickel oxide (Li(Ni—Co—Al)O2), lithium composite metal oxide (Li(Ni—Co—Mn)O2) and the like are used as other layered cathode active materials. Further, spinel lithium manganese oxide (LiMn2O4) and olivine iron phosphate lithium (LiFePO4) having a low cost and superior stability also attract much attention.
Regarding a method for synthesizing these substances, industrial lithium cobalt oxide (LiCoO2) is synthesized using a solid synthesis method in which raw materials are mostly synthesized by solid reaction at 800 to 1,000° C. C. The reason is that a solid method generally uses cheap raw materials such as oxides, hydroxides or carbonates of metals is suitable for mass-production and exhibits substantially superior cycle properties.
In general, a solid method comprises mixing lithium and cobalt as raw materials to prepare a pellet of a mixture, thermally treating the pellet in the air at 800 to 1,000° C. for 20 to 24 hours, and grinding the pellet. Also, the ground oxide is prepared into a pellet again and is then repeatedly subjected to thermal treatment and grinding processes.
However, as can be seen from the synthesis method, since this method requires solid reaction of raw materials, a synthesis temperature should be high. Diffusion distance between raw materials is large, thus causing an increase in thermal treatment time. Furthermore, thermal treatment and grinding processes should be performed several times in order to control homogeneity during synthesis.
In order to solve these problems of solid method, a variety of synthesis methods such as low-temperature synthesis, liquid reaction of raw materials or a method for synthesizing lithium metal oxide including preparing a homogeneous precursor from a liquid and thermally treating the precursor are researched.
Researched representative solid methods include a sol-gel method, a co-precipitation method, hydrothermal synthesis, ion exchange reaction under hydrothermal conditions, mechanical alloying, ultrasonic spray pyrolysis, reflux reaction and the like.
A variety of methods for preparing multi-component metal oxide-based cathode active material precursors were suggested. However, co-precipitation using multi-component metal salts such as nickel, cobalt, manganese and aluminum as starting materials is considered to be the most economic and practically applicable method.
However, co-precipitation has disadvantages in that it is difficult to prepare particles having a uniform size, since multi-component precursors prepared by co-precipitation contain a great amount of fine particles with a wide particle size distribution due to long retention time in a continuous stirred-tank reactor (CSTR), and the precursors contain a great amount of alkali salts as by-products produced during co-precipitation.
The inventors of the present invention researched an apparatus and a method for preparing a cathode active material precursor for lithium secondary batteries using co-precipitation for preparation of cathode active material precursors for lithium secondary batteries, capable of obtaining uniform particles and performing processes with superior reproducibility. The present inventors discovered that aggregation of crystal particles for a short reaction period of time is facilitated and cathode active material precursors for lithium secondary batteries can be prepared in the form of uniform particles by using a double cylindrical rotation crystallizer for preparing the cathode active material precursors for lithium secondary batteries according to the present invention and the method for preparing the cathode active material precursors using the apparatus. The present invention has been completed based on this discovery.